The ability to move the fingers relatively independently of one another enables humans and non-human primates to manipulate diverse objects in the environment and to execute an immense variety of movements and gestures. Lesions of the motor cortex, such as those associated with stroke, give rise to persistent deficits in the ability to move the fingers individually even though the capacity to control other limb segments recovers. Such severe impairment in finger dexterity following stroke might arise because of the unusually complex coordination required among multiple muscles to produce even simple finger movements. However, little is known about how the central nervous system orchestrates ensembles of muscles to 3roduce movements of the fingers and hand. The broad goal of this project, therefore, is to identify the neural mechanisms whereby the activities of multiple muscles are coordinated in the elaboration of finger movements. Specifically, cross-correlation analysis of the firing times of motor units located in different hand muscles of human subjects will be used to estimate the extent to which coordinated activity among muscles is due to divergence of descending pathways providing common synpatic input across sets of motor nuclei. The specific aims are designed to identify how hand muscles are assembled into functional groups for four widely different forms of synergistic activity that underlie: 1) the coordination of compartments of multi-tendoned muscles that insert upon multiple digits, 2) the coordination of separate muscles that insert upon the same digit, 3) the coordination of separate muscles that insert upon different digits yet are used habitually together for specific functions, and 4) the coordination between prime movers of digits and muscles whose activity provides joint stabilization necessary for effective digit movements. These studies will expand our knowledge of the mechanisms by which the brain controls voluntary movements and may provide important insights into the causes of motor dysfunction associated with stroke.